136 ESSENTIALS OF ZOOLOGY 



The medulla oblongata is the most posterior division of the brain. 

 Its anterior portion is somewhat covered by the cerebellum, and ven- 

 trally it has only the pons between it and the midbrain. The cavity 

 of the medulla (fourth ventricle) is ventral to the posterior portion 

 of the cerebellum and is covered by a vascular membrane which is a 

 part of the chorioid plexus. The respiratory center and certain of 

 the other reflex centers are located in this division. It serves also as 

 the important pathway for the bundles of nerve fibers extending from 

 other parts of the brain to the spinal cord. 



The brain developed as a specialized anterior portion of the em- 

 bryonic neural tube and has retained the hollow form. This original 

 cavity is modified into spaces within the principal divisions and lobes 

 in the form of ventricles. The first two, the lateral ventricles, are lo- 

 cated one in each of the cerebral hemispheres, and extend anteriorly 

 into the olfactory lobes. Posteriorly they join the third ventricle of 

 the diencephalon by way of the foramen- of Monro. This third ven- 

 tricle extends ventrally into the infundibulum, which is a downpush- 

 ing in the floor of the diencephalon. A narrow channel, the cerebral 

 aqueduct (aqueduct of Sylvius), leads posteriorly from the third ven- 

 tricle through the midbrain and ventrally to the cerebellum to join 

 the broad fourth ventricle of the medulla oblongata. The fourth ven- 

 tricle continues posteriorly into the narrow cavity (neurocoele) of 

 the spinal cord. 



Spinal Cord. — This is the division of the central portion of the 

 nervous system which extends through the length of the trunk and 

 into the tail. This elongated cord is in the shape of a somewhat 

 flattened cylinder which tapers to a fine-pointed filum terminale at 

 the posterior end within the anterior portion of the tail. There are 

 two enlargements along the length of the cord. They are the cervical 

 enlargement in the neck-shoulder region, and the sciatic enlargement 

 in the lumbar region. The large nerves supplying the front and hind 

 limbs arise at these levels, respectively. Spinal nerves are distributed 

 in pairs and leave the cord segmentally. The neurocoele is the small 

 continuous cavity through the length of the cord, joining the fourth 

 ventricle at the anterior. The gray matter containing nerve cell 

 bodies surrounds the neurocoele in the shape of the letter H or, per- 

 haps more accurately, in the shape of a butterfly with wings spread. 

 The white matter, which is largely nerve fibers, surrounds the gray 

 matter. These fibers extend lengthwise throuerh the cord and connect 



